THE ZOOM FAMILY OF GROUNDWATER MODELSThe ZOOM family of numerical groundwater models consists of the saturated groundwater flow model ZOOMQ3D, the advective transport particle tracking code ZOOPT and the distributed recharge model ZOODRM. Each of these models has been developed using object-oriented techniques, a programming approach commonly applied in commercial software development but only relatively recently adopted in numerical modelling for scientific analysis.

ZOOMQ3D and ZOOPT have been developed through a tri-partite collaboration between Dr Andrew Spink at the University of Birmingham, the British Geological Survey and the Environment Agency of England and Wales. The distributed recharge model ZOODRM has been developed by the British Geological Survey.

ZOOMQ3DNumerical groundwater flow models have become an essential tool in the solution of hydrogeological problems. Their importance arises because they are the only real means by which testing of hypotheses can be conducted. It is common to produce what is referred to as a conceptual model, a description of the processes believed to be operating in the groundwater system. However, to assess the validity of the concepts it is essential to produce results from a more physically based model which can be compared with field information. If there is a high degree of similarity between physical model and reality, then the conceptual model can be treated with some confidence. Numerical models provide information based on the physics of the supposed processes and are therefore the best approximation to a physical model.

Fundamental difficulties arise in constructing a numerical model which is going to be able to be close to the real aquifer. Amongst them are recognising the true mechanisms and finding an acceptable numerical implementation. The ability to look at behaviour on different scales is also important. While a relatively coarse approximation, based on a grid spacing of hundreds of metres, may be acceptable for regional flow through a large expanse of relatively homogeneous aquifer, this will not be adequate for small-scale local behaviour. Examples such as the response of an abstraction well or the representation of a small stream will not be dealt with accurately on a coarse mesh.

The process of model development and construction is iterative. Components of the conceptual model are frequently found to be inadequate and require changes. The inability to reproduce features from the real aquifer brings these inadequacies to light. Change and the introduction of new processes are fundamental to the development process.

ZOOMQ3D is a numerical model which advances the art of model development on two vitally important fronts. It incorporates a mesh refinement procedure which aids the solution of problems related to scale. This is the first of its contributions. The second is that it uses object-oriented techniques as the basis for the program. Whilst this is well-established in the development of general commercial software, it represents a novel approach to groundwater model structure. It is of considerable value in maintaining the code but it is in changing model behaviour that it holds most promise for modellers. Further, the direct correspondence between computer-based objects and real-world features makes the link between numerical and conceptual models very easy to see, even to those with no programming expertise.

ZOOPT particle trackingZOOPT is the particle tracking code associated with the groundwater flow model ZOOMQ3D. The code enables the definition of steady-state and time-variant path lines in three dimensions. Particles can be tracked in both the forward and reverse directions enabling the rapid definition of borehole catchments, recharge and discharge areas and the visualisation of groundwater flow fields, for example. The program also enables the visualisation of steady-state particle tracks that are based on the node-by-node flows at a specific instant of a time-variant simulation.

ZOODRM distributed recharge modelZOODRM is a distributed modelling code for calculating spatial and temporal variations in groundwater recharge. The model is fully compatible with the groundwater flow model ZOOMQ3D but can be used to produce a time series of recharge for other groundwater modelling codes. The model incorporates a standard Penman-Grindley type soil moisture balance method but also includes procedures for recharge estimation in arid countries, irrigated regions and urban areas.The FAO soil moisture balance method, and a simplified version of this algorithm, are included in the model. The latest version of the code also includes overland flow routing and storage, interaction with lakes and reservoirs, rainfall interception, and a groundwater flow model.